Plastisol molding materials are used to a large extent in automobile construction for the bonding and sealing of joints and beaded flanges of automobile body work sheeting, both internally and externally. In particular, plastisols based on PVC polymers and/or copolymers are used for this purpose. It is known that such plastisol molding materials comprise fine polymer praticles dispersed in a liquid, non-volatile plasticizer. At ambient temperature, the solid polymer particles are insoluble in the plasticizer, but the plasticizer dissolves them at elevated temperatures, resulting in a homogeneous solution of the polymer in the plasticizer, which solidifies on cooling to a more or less rigid film.
The plastisol may contain additional components such as fillers, flow control agents, water-absorbing materials such as calcium oxide, stabilizers, pigments, and bond promoters. The purpose of the bond promoters is to cause a lasting bond between the surfaces of conventional work pieces such as oily steel, galvanised or tinned metal sheets, electrocoated metal sheets, etc. Examples of bond promoters for PVC plastisols are polyaminoamides, polyamines, reaction products of excess polyfunctional amines with monomeric or aligomeric bisphenol-A glycidyl ethers, blocked isocyanates, silanes, mixtures of urotropin with resorcinol, etc. and mixtures thereof.
In automobile constructions, plastisols, especially PVC plastisols, are applied either in the body shop to non-degreased crude metal sheets or, following electrocoating, to primed sheets. If plastisol is applied in the body shop, the gelling operation with subsequent hardening takes place in a pre-gelling furnace, prior to the application of the electrocoat, or in the baking furnace for the electrocoat. In the case of application on the electrocoat, the hardening of the plastisol takes place with the baking of the subsequently applied materials, i.e. fillers, primers, and/or a topcoat. In the majority of cases, where fillers or the topcoat are sprayed on, the lacquering is carried out over the previously applied plastisol, whether gelled or not.
It is frequently found that white and pastel shade coating lacquers yellow at those places at which they are applied to the plastisols. Such undesirable discoloration can occur regardless of whether the plastisols have been gelled before lacquering or the coating lacquer has been applied by the wet-on-wet method to the ungelled plastisol. The latter is the application of several layers without drying or baking in between. Discoloration is seldom visible immediately after baking of the coating lacquer and, in the majority of cases, the discoloration does not appear until after some weeks or months. Yellowing of this type has been observed especially when the sheet pretreatment has been changed from anodic to cathodic electrophoretic dip lacquering and the interior spaces of automobile bodies have not been provided with an intermediate lacquer coating.